Two species of hamsters are being used in studies of the effects of photoperiod (e.g., daylength) on reproduction. Both species show gonadal regression when exposed to short days (e.g., 10 hours of illumination daily). In the Syrian hamster the pineal gland is involved in mediating the gonadal response to short days. Recent studies suggest that melatonin is the pineal hormone which causes gonadal regression. Our current studies are directed toward determining whether an alternation in the pattern of melatonin secretion is responsible for the photoperiodic response. In collaboration with Drs. David Klein and Lawrence Tamarkin (NIH) we are measuring changes in melatonin production in relationship to changes in illumination. We are also continuing to study the effects of daily injections of melatonin on gonadal function. These studies are being conducted in both intact and pinealectomized hamsters kept under a variety of photoperiods.